where we have replaced \(\delta (p^{0})\)  by its covariant form \(\delta (p\cdot u_{1,2})\) 
[1]}. Note that in Refs. [2]}, [3]},
an extra phase factor \(e^{i\mathbf {p}_{T}\cdot \mathbf {b}_{T}}\)  is
included into \(A_{(1)}^{\mu }(p)\)  to describe the dependence on the
impact parameter. Since we have already introduced the dependence
on impact parameters systematically into the differential cross sections
(REF ,REF ), we do not need to add this
phase factor into \(A_{(1,2)}^{\mu }(p)\)  in (REF ).
One can verfiy that our formalism is equivalent to Ref. [2]}, [3]}.
